Download Chapter 13 Other Planetary Systems Why is it so difficult to detect

Chapter 13
Other Planetary Systems
13.1 Detecting Extrasolar Planets
The New Science of Distant Worlds
• Our goals for learning
• Why is it so difficult to detect planets
around other stars?
• How do we detect planets around other
stars?
Why is it so difficult to detect
planets around other stars?
Brightness Difference
• A Sun-like star is about a billion times
brighter than the sunlight reflected from its
planets
• Like being in San Francisco and trying to
see a pinhead 15 meters from a grapefruit in
Washington, D. C.
Special Topic: How did we learn
other stars are Suns?
How do we detect planets around
other stars?
• Ancient observers didn’t think stars were like the
Sun because Sun is so much brighter.
• Christian Huygens (1629-1695) used holes drilled
in a brass plate to estimate the angular sizes of
stars.
• His results showed that, if stars were like Sun,
they must be at great distances, consistent with the
lack of observed parallax.
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Planet Detection
• Direct: Pictures or spectra of the planets
themselves
• Indirect: Measurements of stellar
properties revealing the effects of orbiting
planets
Gravitational Tugs
• Sun’s motion around
solar system’s center
of mass depends on
tugs from all the
planets
• Astronomers around
other stars that
measured this motion
could determine
masses and orbits of
all the planets
Doppler Technique
Gravitational Tugs
• Sun and Jupiter orbit
around their
common center of
mass
• Sun therefore
wobbles around that
center of mass with
same period as
Jupiter
Astrometric Technique
• We can detect planets
by measuring the
change in a star’s
position on sky
• However, these tiny
motions are very
difficult to measure
(~0.001 arcsecond)
First Extrasolar Planet
• Measuring a star’s
Doppler shift can tell
us its motion toward
and away from us
• Doppler shifts of star
51 Pegasi indirectly
reveal a planet with
4-day orbital period
• Current techniques
can measure motions
as small as 1 m/s
(walking speed!)
• Short period means
small orbital distance
• First extrasolar planet
to be discovered
(1995)
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First Extrasolar Planet
Other Extrasolar Planets
Large
planet
mass
Highly
eccentric
orbit
• Planet around 51 Pegasi has a mass similar to
Jupiter’s, despite its small orbital distance
Planet Mass and Orbit Tilt
• We cannot measure an exact mass for a planet without
knowing the tilt of its orbit, because Doppler shift tells
us only the velocity toward or away from us
• Doppler data gives us lower limits on masses
Spectrum during Transit
• Change in spectrum during transit tells us about
composition of planet’s atmosphere
• Doppler data curve tells us about a planet’s mass and
the shape of its orbit
Transits and Eclipses
• A transit is when a planet crosses in front of a star
• The resulting eclipse reduces the star’s apparent
brightness and tells us planet’s radius
• No orbital tilt: accurate measurement of planet mass
Direct Detection
• Special techniques can eliminate light from brighter
objects
• These techniques are enabling direct planet detection
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What have we learned?
Other Planet-Hunting Strategies
• Gravitational Lensing: Mass bends light in
a special way when a star with planets
passes in front of another star.
• Features in Dust Disks: Gaps, waves, or
ripples in disks of dusty gas around stars
can indicate presence of planets.
13.2 The Nature of Extrasolar Planets
• Why is it so difficult to detect planets
around other stars?
– Direct starlight is billions of times brighter than
starlight reflected from planets
• How do we detect planets around other
stars?
– A star’s periodic motion (detected through
Doppler shifts) tells us about its planets
– Transiting planets periodically reduce a star’s
brightness
– Direct detection is possible if we can block the
star’s bright light
What have we learned about
extrasolar planets?
• Our goals for learning
• What have we learned about extrasolar
planets?
• How do extrasolar planets compare with
those in our solar system?
Measurable Properties
• Orbital Period, Distance, and Shape
• Planet Mass, Size, and Density
• Composition
Orbits of Extrasolar Planets
• Most of the detected
planets have orbits
smaller than Jupiter’s
• Planets at greater
distances are harder
to detect with
Doppler technique
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Orbits of Extrasolar Planets
• Orbits of some
extrasolar planets are
much more elongated
(greater eccentricity)
than those in our
solar system
Multiple-Planet Systems
Multiple-Planet Systems
• Some stars
have more
than one
detected
planet
Orbits of Extrasolar Planets
• Most of the detected
planets have greater
mass than Jupiter
• Planets with smaller
masses are harder to
detect with Doppler
technique
• Special techniques can eliminate light from brighter
objects
• These techniques are enabling direct planet detection
How do extrasolar planets compare
with those in our solar system?
Surprising Characteristics
• Some extrasolar planets have highly
elliptical orbits
• Some massive planets orbit very close to
their stars: “Hot Jupiters”
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Hot Jupiters
What have we learned?
• What have we learned about extrasolar
planets?
– Detected planets are all much more massive
than Earth
– They tend to have orbital distances smaller
than Jupiter’s
– Some have highly elliptical orbits
• How do extrasolar planets compare with
those in our solar system?
– Some “Hot Jupiters” have been found
13.3 The Formation of Other Solar
Systems
Can we explain the surprising
orbits of many extrasolar planets?
• Our goals for learning
• Can we explain the surprising orbits of
many extrasolar planets?
• Do we need to modify our theory of solar
system formation?
Revisiting the Nebular Theory
• Nebular theory predicts that massive
Jupiter-like planets should not form inside
the frost line (at << 5 AU)
• Discovery of “hot Jupiters” has forced
reexamination of nebular theory
• “Planetary migration” or gravitational
encounters may explain “hot Jupiters”
Planetary Migration
• A young planet’s
motion can create
waves in a planetforming disk
• Models show that
matter in these waves
can tug on a planet,
causing its orbit to
migrate inward
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Gravitational Encounters
• Close gravitational encounters between two
massive planets can eject one planet while
flinging the other into a highly elliptical
orbit
• Multiple close encounters with smaller
planetesimals can also cause inward
migration
Do we need to modify our theory
of solar system formation?
Orbital Resonances
• Resonances between
planets can also cause
their orbits to become
more elliptical
Modifying the Nebular Theory
• Observations of extrasolar planets have
shown that nebular theory was incomplete
• Effects like planet migration and
gravitational encounters might be more
important than previously thought
What have we learned?
Planets: Common or Rare?
• One in ten stars examined so far have
turned out to have planets
• The others may still have smaller (Earthsized) planets that current techniques cannot
detect
• Can we explain the surprising orbits of
many extrasolar planets?
– Original nebular theory cannot account for
“hot Jupiters”
– Planetary migration or gravitational
encounters may explain how Jupiter-like
planets moved inward
• Do we need to modify our theory of solar
system formation?
– Migration and encounters may play a larger
role than previously thought
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13.4 Finding New Worlds
How will we search for Earth-like
planets?
• Our goals for learning
• How will we search for Earth-like planets?
Transit Missions
• NASA’s Kepler
mission is scheduled
to begin looking for
transiting planets in
2008
• It is designed to
measure the 0.008%
decline in brightness
when an Earth-mass
planet eclipses a Sunlike star
Direct Detection
• Determining whether
Earth-mass planets
are really Earth-like
requires direct
detection
Mission concept for NASA’s
Terrestrial Planet Finder (TPF)
• Missions capable of
blocking enough
starlight to measure
the spectrum of an
Earth-like planet are
being planned
Astrometric Missions
• GAIA: A European mission planned for 2010 that
will use interferometry to measure precise motions
of a billion stars
• SIM: A NASA mission planned for 2011 that will
use interferometry to measure star motions even
more precisely (to 10-6 arcseconds)
What have we learned?
• How will we search for Earth-like planets?
– Transit missions will be capable of finding
Earth-like planets that cross in front of their
stars (Kepler to launch in 2008)
– Astrometric missions will be capable of
measuring the “wobble” of a star caused by an
orbiting Earth-like planet
– Missions for direct detection of an Earth-like
planet will need to use special techniques (like
interferometry) for blocking starlight
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